MOO 1/2
MOM
GalCiv1
BG1/2
NWN
WC2/3
WoW
Homeworld
XCOM (the old one not the new one)

Forgot:
KOTOR
Civilization
Red Alert

Each of these was ground-breaking somehow. XCOM was an usual combination of base building, strategy, research (RPG feel) and turn based game, highly unusual. Civ was superbly well balanced and had an authentic feel. RA was just a great, fun game, continuing a long line of resource gather/build base/build unit type games. (The original of the series incidently is dune2, a good game too). KOTOR was an amazingly well done RPG game set to SW. Homeworld was the first real 3D tactical game of any kind that was decent (that i knew about) - another game that really took advantage of 3D was descent and the WC series. MOO was one of the best 4x games ever created, superbly balanced, quick and with depth.

Fallout 2Homeworld: Cataclysm. (The expansion, inspite of its name, was actually way better than the original, and devs of the original didn't learn anything at all from it and they released Homeworld 2 which was an annoying clusterfuck of a strategy game, which had a faster gameplay than most FPSs on the market, making it enjoyable only for people with twitch reflexes and really fast thinking). Planescape: TormentGothic 1 (Gothic 2 was meh and Gothic 3 was crap as well as all the horrors that followed from Piranha Bytes)

Of course Half Life 1 & 2, Oblivion and Skyrim are among the top, honourable mentions to WoW and I'm biased to say Dark Souls: Prepare to Die even though it was a shoddy port, with a gamepad and graphics fix it's amazing.

I base a "good game" on how it provokes different emotions, that’s my measuring stick at least 

1. Baldur’s Gate 1 (For its sense of adventure and overall story)
2. Spec Ops: The line (The only game I’ve played that isn’t fun but provokes feelings such as regret, guilt and sadness)
3. Mass Effect Series (For its epic storyline and epic feel)
4. Dragon Age 1 (Same as above)
5. Skyrim (Gives me the feel of adventure)

I don’t count games that are simply addictive (WoW) because that is not a good game mechanic in my opinion. (good for making the dev money)